The Friction Stir Welding (FSW) process is a solid-state based joining process, which makes it possible to weld a wide variety of materials (Aluminum, Copper, Stainless Steels, etc.) to themselves and to weld various combinations (e.g. aluminum alloys 6xxx/5xxx, 2xxx/7xxx, etc.) to each other. The process is based on “plunging” a rotating FSW tool into the joining area. The rotating friction stir welding tool heats the workpiece(s) by friction, so the material becomes plasticized and flows around the axis of the tool due to shear caused by the tool.
FIG. 1 shows a prior art friction stir welding tool 10. The tool 10 includes a pin 12 which, preferably, is threaded. The shank 18 is for gripping in a chuck or collet of the friction stir welding machine. The tool 10 also includes shoulder 14, having workpiece engaging surface 16, which is for preventing material from flowing upwardly, out of the joint being formed. The tool is rotated in the direction such that the threads 13 on pin 12 push plasticized material downwardly into the joint. A setup for friction stir welding is shown in FIG. 2.
The setup includes a rotating chuck or collet 11 which grips shank 18 of tool 10. The setup also includes a moveable carriage (bed) 19 to which the workpiece(s) 111 is/are clamped. An anvil 115 underlies workpieces 111. The carriage 19 is translated relatively to the chuck or collet 11, so the pin 12 progresses along the joint 114 which is to be welded. Conversely, the relative motion of the FSW tool holding chuck or collet can also be achieved by keeping the carriage 19 and workpiece(s) 111 stationary and moving (or translating) the welding head relative to them.
FIGS. 3-7 illustrate the process of beginning a friction stir welding process. Rotary motion is imparted to tool 10 having pin 12 as shown in FIG. 3. Then, while tool 10 is rotating, it is brought down to the workpieces(s) 111 as shown in FIG. 4. The tool 10 is then pressed downwardly so the pin 12 contacts the workpieces, preferably at joint 114 and begins heating them locally by friction, and the heat plasticizes the workpiece material. The tool is then further pressed downwardly into the workpieces, as shown sequentially in FIGS. 5, 6, and 7.
The heat due to the friction causes plasticized material 112 of the workpiece(s) 111 to soften and flow around the axis of the pin 12. In FIG. 7, the pin 12 is plunged almost entirely into the workpiece(s) 111. Typically, an anvil 115 lies below the workpiece(s) 111 to counteract the downward “plunging” (or Fz) force imparted by the tool holding chuck onto the joint area and maintain a smooth surface on the underside of the workpiece(s) 111.
Once the pin 12 has been plunged into the workpiece(s) 111, the bed 19 is translated, so the pin 12 is moved relative to the workpiece(s) 111 along the joint to be welded. As the plasticized material cools behind the pin 12, it coalesces into sound metallurgical bonds.
FIG. 8 illustrates in greater detail the principal aspects of a prior art FSW tool 10. Tool 10 includes a shank 18, a pin 12 and a shoulder 14. Shank 18 may have a flat 21. Shoulder 14 has a workpiece engaging surface 16. Pin 12 has threads 13 and flats 15. When tool 10 is in use, it is rotated in the direction that will cause plasticized material engaged by threads 13 on pin 12 to move downwardly, into the workpiece(s) 111. The flats 15 serve to reduce the torque needed to rotate the pin 12. The workpiece engaging surface 16 of the shoulder 14 has a spiral thread (or scroll) 17 which tends to cause plasticized material to flow inward radially, toward the base of pin 12, when the tool is rotated in the direction which is appropriate for threads 13 on pin 12.
FIG. 9 illustrates an integral (or monolithic) FSW tool 20. The pin 22, shank 28 and shoulder 24 are integrally formed. Good concentricity is obtained between the shank 28 and the pin 22. The disadvantage of this design is that it may be desirable to make the pin 22 of a material having preferred properties, which may not be needed for the shoulder 24, but which is very expensive. For an integrally formed tool, much of that material would be wasted on the shoulder 24. Also, the material may be available only in small diameter form, insufficient to form the shoulder 24.
One prior art solution to this problem is to make the pin out of a material (e.g. MP159) that is different from the material out of which the shoulder and shank are made (e.g. H13). The traditional way of designing such a composite FSW tool is to “consolidate” the tool's shoulder with the shank into one piece, and then insert the pin into it.
FIGS. 10 and 11 illustrate a prior art tool 30 having a shoulder 34 which is integral with the shank 38. The tool has a pin 32 which is held by one or two setscrews 39. There may be some clearance 33 between the pin 32 and the shoulder 34. It is very difficult to obtain good alignment between the pin 32 and the shoulder 34/shank 38 with this design. Hence, pin 32 tends to be eccentric and to wobble as it turns during friction stir welding of the workpiece(s). This introduces cyclic loads on pin 32 that may cause it to break. This is especially true when joining strong, hard materials (e.g. 7055, Stainless Steel, Titanium) and/or relatively thick parts (e.g. 45 mm).
These eccentricities, which lead to more pronounced vibrations and cyclic loading of the pin 32 during welding, often shorten the life of the pin 32. This is typically manifested by breakage of pin 32 near its base, near the shoulder 34 of the tool 30. These eccentricities are caused by:                (a) The intentional clearance 33 designed between the pin 32 and the hole in the shoulder 34 to accept it (FIG. 11).        (b) Uneven and unrepeatable tightening of the set-screws 39, which varies the clearance between the pin and the hole that accepts it in the shoulder.        (c) Normal compounding of machining tolerances of the pin and shoulder/shank.        (d) Variations in placement of the shank within the FSW machine's chuck or collet.        
Accordingly, there is a need for FSW tools which reduce shank-pin misalignment and hence are more resistant to breakage.